One of the most frustrating games ever for me is Wargame: European Escalation
I just cannot figure out any way to flush out well placed infantry in the woods or in towns(worse). There is just no way to detect these fuckers. If I use land recon units, they simply don't see infantry in towns unless they come close, within range of the hidden infantry in which case the recon unit gets owned.
The recon helicopter, when set to high altitude does a slightly better job, but again, often has to come a bit closer, and constantly hover there to spot for arty which is like the only unit that can really effectively kill the hidden infantry squad. I am assuming that a gunship heli would also be able to do the job, but again a recon heli would be also needed to spot for it which is just too much work overall.
In the last mission, just TWO small AT infantry squads were able to hold my heavy tank advance for a very long time, frustrating me beyond belief.
Also, Baldur's Gate and Neverwinter Nights was extremely frustrating for me.